Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards. Non-CDMA systems include the AMPS and GSM systems.
A CDMA system may be deployed in a portion of spectrum that is divided into multiple channels. Base stations, or access points, within the system may support communication on one or more channels. When a mobile station, or access terminal, locates an access point, it can monitor broadcast information relevant to that access point. An access point may broadcast a list of channels that it supports. Procedures are known whereby an access terminal selects one of the supported channels of an access point with which to communicate. One example is for the access terminal to use a hash function, with the number of channels supported and an access terminal identifier, such as the International Mobile Subscriber Identification (IMSI), as inputs to the hash function. Such procedures allow the access terminals, many of which will be in idle mode, to be distributed evenly, on average, over the access point's available channels.
On occasion, perhaps as an access terminal travels throughout a system, it may be desirable to handoff to a neighboring access point. To facilitate handoff, an access point may broadcast a list of neighboring access points, known as the neighbor list. The neighbor list may contain, along with each access point, a channel supported by that access point. Thus, an access terminal can monitor the received signal strength of the access point it is currently communicating with (whether in the idle state or the traffic state), as well as the received signal strengths of one or more neighboring access points, as directed by the neighbor list. An access terminal can hand off to a neighbor based on the relative strength of the received signals. In some systems, an access terminal will require that a neighboring access point's signal exceed a threshold before handing off to that access point, to minimize repeated handoffs back and forth between two access points due to minor fluctuations in relative received signal strength.
One access point may support a set of channels that is different from the set of channels supported by a neighboring access point. In general, it is desirable to reserve as much of the capacity of a CDMA system for voice or data communication, by minimizing the amount of capacity used for non-traffic signaling. Therefore, the neighbor list is transmitted with a single channel supported for each neighbor, rather than the complete list of supported channels. An access terminal, when handing off to a new access point, monitors the broadcast of supported channels on the new access point, and then determines the appropriate supported channel, or assigned channel, on which to communicate. For example, the hash function described earlier can be used.
While certain conditions may cause uniform interference to signals received from an access point, some interference is frequency dependent. Therefore, while the received signal strength in the channel associated with a neighboring access point in the neighbor list may be suitable for a handoff, the received signal strength of the channel with which the access terminal will ultimately communicate may be unsuitable due to frequency dependent fading. In such a situation, an access terminal may hand off from a first access point to a second access point included in the neighbor list, determine the proper channel on which to communicate, determine that the channel is not sufficient, and thus return to the first access point (or perhaps a third access point). If the conditions have not changed significantly, the access terminal will make the same handoff decision as before, only to find the appropriate channel on the second access point is still unsuitable. Until conditions change, the access terminal may be caught in an endless loop, cycling from one access point to another and back again. Applying a threshold as described above, during such a frequency dependent fade, will not prevent the loop from forming. There is therefore a need in the art for avoiding handoff to a channel impaired due to frequency dependent fading, while minimizing idle mode signaling.